Electronic coherence dynamics in trans-polyacetylene oligomers

TL;DR

This study investigates electronic decoherence in trans-polyacetylene oligomers by simulating coupled electronic and vibrational dynamics, revealing that decoherence occurs within tens to hundreds of femtoseconds depending on oligomer size and initial superposition.

Contribution

It provides a detailed quantum-classical simulation framework for understanding electronic decoherence timescales in conjugated polymer oligomers, highlighting size and initial state effects.

Findings

Decoherence occurs in tens of femtoseconds for long oligomers.

Decoherence times vary from 50 to 250 fs depending on superposition.

Long-lived superpositions involve minimal population transfer and harmonic vibronic dynamics.

Abstract

Electronic decoherence processes in trans-polyacetylene oligomers are considered by explicitly computing the time dependent molecular polarization from the coupled dynamics of electronic and vibrational degrees of freedom in a mean-field mixed quantum-classical approximation. The oligomers are described by the SSH Hamiltonian and the effect of decoherence is incorporated by propagating an ensemble of quantum-classical trajectories with initial conditions obtained by sampling the Wigner distribution of the nuclear degrees of freedom. The decoherence for superpositions between the ground and excited and between pairs of excited states is considered for chains of different length, and the dynamics is discussed in terms of the nuclear overlap function that appears in the off-diagonal elements of the electronic reduced density matrix. For long oligomers the loss of coherence occurs in tens of femtoseconds. This timescale is determined by the initial decay of the nuclear overlap and by the decay of population into other electronic states, and is relatively insensitive to the type and class of superposition considered. By contrast, for smaller oligomers the decoherence timescale depends strongly on the initially selected superposition, with superpositions that can decay as fast as 50 fs and as slow as 250 fs. The long-lived superpositions are such that little population is transferred to other electronic states and for which the vibronic dynamics is relatively harmonic.